Growing semiconductor crystalline materials

ABSTRACT

Semiconductor crystalline materials, e.g. silicon, GaAs, are grown from a melt, e.g. using the Czochralski technique where a seed crystal is dipped into the melt then slowly withdrawn. Rotation of the growing crystal (6) is partly responsible for convective flows within the melt (5). Convective flows are reduced while radial uniformity is improved by subjecting the crystal/melt interface to a shaped magnetic field. This magnetic field is rotationally symmetrical about the axis of crystal rotation, with a component of field parallel to this axis that is less than 500 gauss, preferably less than 200 gauss, with a value above 500 gauss at other parts of the melt. The field may be produced by two superconducting magnet coils (21, 22) spaced apart and arranged co-axially with the axis of crystal rotation.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a method and apparatus for growingsemiconductor crystalline materials in which a melt of the growingcrystal is subjected to a magnetic field.

2. Discussion of Prior Art

Electronic devices are fabricated on wafers of semiconductor crystals.The properties of the semiconductor are heavily dependent on thespecific impurities present in the wafers. Not all impurities aredeleterious, and successful fabrication of electronic devices requiresthe control of the distribution and concentration of the impurities. Formost electronic devices the starting material comprises a slice of wafercut from a single crystal of the semiconductor, commonly either siliconor gallium arsenide. The crystal is in turn pulled from a melt. Anobject of this invention is an improvement to the techniques for thegrowth of semiconductor crystals which leads to improved homogeneity ofcertain impurities in the crystal.

Semiconductor crystals are for the most part grown from melts of thesemiconductor in a crystal pulling apparatus. To control the electricalproperties of the crystals, small quantities of specific impurities ordopants are added to the melt. One of the major problems in theproduction of crystals is to achieve control over the uniformity andconcentration of dopants within the crystal.

During the growth process differences in composition or temperaturebetween differing regions of the melt lead to density variations in themelt which can in turn have undesirable effects on the incorporation ofimpurities within the growing crystal and lead to the formation oflocalised fluctuations of impurity concentrations known as striae.

SUMMARY OF THE INVENTION

Often, during crystal growth, the crystal and sometimes other parts ofthe apparatus are rotated to provide uniformity within the growingcrystal. These rotations are also partly responsible for setting upconvective flows within the melt. The form of these convective flowsgreatly affects the quality of the growing crystal and can help topromote uniformity of the crystal. One effect of the rotation of thecrystal is to set up a convective motion in the melt ahead of thegrowing crystal as a result of the centrifugal pumping action of therotating crystal. Such crystal rotation driven flow is generallydesirable as it results in an improved radial uniformity to the crystal.

In recent years much interest has been displayed in the potential fordamping of the convective flows in the melt through the application ofexternally applied magnetic fields. The melts of semiconductors aregenerally good electrical conductors and it is well established that itis possible to damp the convective flows within the melts by theapplication of a static magnetic field to the melt. This can lead togreatly differing patterns of incorporation of impurity within thecrystal.

To date two geometries have been widely reported. In the so called axialsystems a coil is placed co-axially with the rotation axis of thecrystal so as to generate a field in the melt predominantly directedparallel to the rotation axis. In some modifications of this methodseveral coils are used, all co-axial with the rotation axis, and withcurrent circulating in the same sense in each coil. The objective ofsuch an arrangement is to provide a much more uniform field over themelt volume. The second geometry is to use a magnet oriented so as togenerate a field transverse to the growth direction of the crystal. Sucha geometry destroys the rotational symmetry which is generallyconsidered desirable during crystal growth.

While the application of static magnetic fields has been shown to becapable of damping the convective flows within the melt and so reducingthe striae, the quality of the crystal may be degraded in otherrespects. For example in UK Patent GB 2,163,672 A it is proposed thatthe reduction in heat transport through the melt brought about by thereduced convection can lead to unacceptably high crucible walltemperatures during growth. To avoid this problem the patent suggeststhat the field be shaped so as to create a low field region in the meltaway from the crystal melt interface while retaining a high field closeto the crystal.

Another problem which is frequently encountered during growth under amagnetic field is that the radial uniformity of the incorporation of thedopant is degraded. In some cases this can arise as a result of themagnetic field near the crystal/melt interface which damps the crystaldriven flows. During the growth of a crystal which is being rotated inthe melt it is well known that, as first demonstrated by Burton, Primand Schlicter, the rotation sets up a centrifugally pumped convectiveflow in the melt. During the growth, the centrifugal pumping action actsto remove the impurities which are rejected. The magnetic field may actto damp this pumping action so leading to an excess build up ofimpurities ahead of the growing crystal. Radial variations in the mixingof this liquid with the bulk of the melt will lead to radial variationsin the incorporation of these impurities.

In addition to reducing the striae in the crystal, a second advantagewhich may be gained by the application of a magnetic field to the meltis to reduce the contamination of the melt by the crucible. In the caseof silicon growth it has been shown that the application of a transversefield to the melt can under some circumstances, reduce the incorporationof oxygen into the growing crystal.

The present invention improves the radial uniformity of a crystal grownunder a magnetic field by shaping the field so as to reduce the dampingaction on the crystal rotation driven flows. The configuration of thefield is such that the flows in the melt responsible for transport ofimpurities from the crucible to the crystal are damped, so reducing thecontamination of the crystal by the crucible.

According to this invention a method of growing semiconductorcrystalline materials comprises the steps:

providing in a crucible a melt of the material to be grown as a crystal,

dipping a seed crystal into the melt,

withdrawing and rotating the seed crystal from the melt,

maintaining a temperature gradient between the melt and seed crystal,

whereby a single crystalline material is grown from the melt,Characterised by:

providing a magnetic field that is substantially rotationallysymmetrical about the axis of rotation of the crystal and with acomponent of field parallel to the axis of crystal rotation that is lessthan 500 gauss at the interface between growing crystal and melt, with avalue of magnetic field above 500 gauss at other parts of the melt,

and maintaining this distribution of magnetic field during growth of thecrystal,

whereby forced convective flows in the melt adjacent the crystal/meltinterface are substantially undamped, whilst convective flows in otherparts of the melt are damped.

Preferably the axial component of magnetic field at the crystal/meltinterface is less than 200 gauss over a region greater than the area ofthe crystal.

During growth the crucible may be rotated about its axis and raised soas to maintain the correct relation between the crystal melt interfaceand the magnetic field.

In a modification the magnet assembly may be lowered during growth tomaintain the correct relation between the crystal melt interface and themagnetic field.

According to this invention apparatus for growing semiconductorcrystalline materials comprises:

a chamber containing an electric heater,

a crucible arranged within the heater for containing a charge of thematerial to be grown,

a pull member seed crystal onto which a crystal may be grown,

motors for rotating and axially moving the pull member,

means for controlling the heater and pull member movement to pull acrystal from a melt established in the crucible, characterised by:

means for providing a magnetic field in the melt, said magnetic fieldhaving a plane that is rotationally symmetric about an axis of rotationof the pull member with a component that is less than 500 gauss in adirection along said axis, the magnetic field away from the plane havinga magnetic value greater than 500 gauss,

means for locating an interface between crystal and melt at said planeso that melt adjacent the interface is subjected to an axial componentof magnetic field less than 500 gauss, and melt away from the interfaceis subject to a magnetic field above 500 gauss, and

means for maintaining the relative position of the crystal/meltinterface in said magnetic field plane during growth of the crystal.

The pull member may be a rod, ball chain, flexible wire, or similarsupport.

The magnetic field may be provided by two or more superconductingring-shaped magnets spaced apart and arranged coaxially with the axis ofpull rod rotation.

Alternatively the magnetic field may be generated by conventionalresistive electro magnets.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described, by way of example only, withreference to the accompanying drawings of which:

FIG. 1 a view of a Czochralski puller;

FIG. 2 is an enlarged view of part of FIG. 1 showing the junctionbetween growing crystal and melt;

FIG. 3 is a map of the magnetic field contours within a crucible for theapparatus of FIG. 1;

FIG. 4 is a graph showing resistivity variation across a crystal for acrystal grown under high axial magnetic field, and for a crystal grownaccording to this invention;

FIG. 5 is a graph showing the oxygen content of crystals grown underdifferent magnetic fields.

DETAILED DISCUSSION OF THE PREFERRED EMBODIMENTS

The Czochralski growth apparatus FIGS. 1, 2 comprises a chamber 1containing a silica crucible 2 surrounded by a picket fence type ofresistance heater 3. The crucible 2 is mounted for rotation and verticalmovement by a motor 4. Inside the crucible 2 is an amount of siliconmelt 5 from which a crystal 6 is pulled by a seed crystal 7 attached tothe bottom of a pull, chain, wire or rod 8. This pull rod 8 is bothrotated and moved vertically by a motor 9. Electrical power is fed tothe heater from a heater power supply 10. Argon gas is fed from a bottle11 via a valve 12 to the inside of the chamber 1. Gas is pumped from thechamber 1 by a pump 13 which maintains the pressure below atmosphere,typically at 20 Torr. Cooling water is circulated through a jacket 14surrounding the chamber 1 and fed from a water supply 15. Two photodiodes 16, 17 of adjustable spacing, are arranged to receive lightreflected off the junction between growing crystal 6 and melt 5. Thiscan be observed as a bright ring. The amount of light received by thephoto diodes 16, 17 is used in a feed back loop to a control logic unit18. This control unit may be a programmed computer or analog controlsystem and controls the motors 4 and 9, heater supply 10, pump 13, andcoolant supply 15.

FIG. 1 additionally has two super-conducting coils 21, 22 of 520 mmcentre diameter spaced 580 mm apart centre to centre surrounding thechamber 1. These coils 21, 22 are cooled by liquid helium, fed from a Hesupply 23 to provide a super conducting magnet. Additional cooling isfrom liquid nitrogen, fed from a N supply 24, the complete coils andcooling He, N being encased in a vacuum chamber 25. A magnetic field isset up in the coils 21, 22 by power from a power supply 26.Conventionally this is achieved by heating a small part of each coil tomake it resistive and applying power to the remainder of the coil. Whenthe field is at its desired strength the complete coil is madesuperconducting so that the magnetic field is maintained withoutexternal power supply.

FIG. 3 shows measured magnetic field contours inside a crucible 20.3 mm(8 inch) diameter and 10.15 mm (4 inch) deep when maximum power wasapplied to the coils 21, 22. The coils 21, 22 can be operated at lowerpower with a consequential reduction in magnetic field strength.

The coils 21, 22 are arranged coaxially with the rotational axis of thegrowing crystal 6, pull rod 8, and crucible 2. Electric current isarranged to be passed into the upper coil 21 in a clockwise direction,when seen from above, and into the lower coil 22 in an anticlockwisedirection. This results in a radial magnetic field in a plane 28perpendicular to the axis of the coils 21, 22. In this plane 28 theamount of magnetic field component parallel to the crystal 6 axis, i.e.perpendicular to the plane 28, is less than 200 gauss. Elsewhere themagnetic field will have radial and axial components which in placesexceeds 1000 gauss. By way of comparison the earth's magnetic field istypically about 1 gauss. The centre of the plane 28 has zero radialmagnetic field. It is important that the plane 28 is correctlypositioned relative to the growing crystal. As shown more clearly inFIG. 2 the plane 28 passes through the solid/melt interface 27 withabout half of the interface area above and half below the plane 28.During growth of a crystal 6 the level of melt 5 in the crucible 2drops. To compensate for this the crucible 2 is slowly raised by themotor 4 to maintain the solid/melt interface 27 in the plane 28.

Operation to grow a Si single crystal of 75 mm nominal diameter will nowbe described. A charge of 6 kgs of Si doped to contain about 10 atoms ofphosphorus is arranged in the crucible 2. The chamber 1 is pumped downto about 0.1 Torr to remove air and other contaminants, then argon isadmitted from the bottle 11 whilst the pump 13 maintains the pressure atabout 20 Torr. An electric current is passed through the heater 3 tomelt the charge in the crucible 2 and provide a melt 5, a typicaltemperature is 1460 C.

The seed crystal 7 is rotated and lowered to make contact with the melt5. After adjustments to the temperature of the melt 5, the seed 7 israised at a controlled rate and with a controlled rotation. Bycontrolling the rate of travel of the seed 7 and the temperature of themelt 5 a crystal 6 of silicon of the required diameter may be grown.Initially the growth is increased from the seed diameter until therequired 75 mm is reached. An increase in pull rate is needed to grow ata constant diameter. Thereafter the diameter is maintained constantuntil the end of the run when the melt 5 is depleted. Once the requireddiameter is reached the photo diodes 16, 17 are adjusted to maintain therequired value as variations in diameter are detected as variations indiode output.

Once growth has been established at the required 75 mm diameter, anelectric current is passed through the super-conducting coils 21, 22.The result is that the crystal 6 grows in a small axial magnetic fieldthat has insufficient strength to damp the beneficial crystal drivenflows in the adjacent melt. However, the magnetic field away from themelt/crystal interface 27 has sufficient strength to damp out theconvective flows in the main body of the melt 5. The quality of thecrystal 6 is therefore improved through the reduction of the undesirableflows in the bulk of the melt 5 and the retention of the beneficialflows ahead of the growing crystal 6.

As the crystal 6 continues to grow the control unit 18 monitors theoutput of the photo diodes 16, 17 and makes appropriate adjustments tothe heater 3 temperature and crystal pull rate to maintain growth at thedesired diameter. Also the position of the crucible 2 is adjusted by themotor 4 during growth to maintain the interface 27 in the radial fieldplane 28.

Growth is continued until most of the melt has been used. The pull rateis then increased to taper off the crystal diameter and terminategrowth. Heater power is stopped the crystal and apparatus allowed tocool. After growth is completed the crystal is removed from theapparatus and ground to the exact diameter required. The crystal is thensliced, edge rounded, chemically and mechanically polished and heattreated ready for the manufacture of electronic integrated circuits.

The principal detailed above may be applied to the growth of anysemiconductor crystal from the melt in which the crystal is rotated. Forexample, the method may be applied to the growth of silicon crystals byeither the Czochralski or Float Zone techniques or to the growth ofgallium arsenide by the liquid encapsulated Czochralski method.

In the Float Zone technique a zone of melted crystal is gradually movedalong a rotating crystal; the material recrystallising behind the movingmelt zone. The recrystallising interface is subjected to the samemagnetic fields as that in FIGS. 1, 2, 3.

The crucible may be moved during crystal growth to maintain the requiredrelative position of crystal/melt interface and magnetic field.Additionally the crucible may be rotated.

Alternatively the means for providing a magnetic field may be movedduring growth to maintain the required relative position of crystal/meltinterface and magnetic field.

One measure of the uniformity of a crystal is its electrical resistivityprofile across a wafer. This is shown by FIG. 4 for a phosphorus doped,silicon crystal cut normally from a crystal at a position along thelength corresponding to the solid fraction (G)=0.41. The sample shown bycurve A was prepared in a Czochralski growth apparatus in which the meltwas immersed in a magnetic field whose axial components exceeded 1500gauss in the plane of the crystal melt interface. The magnetic field hasdamped the flows driven by the crucible rotation such that there arelarge radial variations in the dopant concentration and henceresistivity.

The sample shown in curve B was prepared in a Czochralski pullerequipped with superconducting solenoids operated such that the componentof magnetic field in the plane of the crystal melt interface did notexceed 200 gauss but with the field exceeding 1500 gauss in some otherparts of the melt. The magnetic field was designed to retain therotational symmetry of the system. The radial profile of the resistivityis very much improved since the field has been shaped to minimise thedamping of the crystal rotation driven flows.

The effect of varying the magnetic field strength is shown in FIG. 5.The vertical axis shows oxygen content and the horizontal axis shows thefraction of the original crucible melt and hence represents lengthmeasured along a grown crystal. Only about 0.8 of a melt is useful; theremaining 0.2 contains impurities and is often left in the crucibleafter crystal growth. A grown crystal is sliced and its oxygen contentmeasured at different positions along its length. The broken line (curveC) represents oxygen content variation with no applied magnetic field.As the applied magnetic field is increased to 0.35 (curve D), 0.5 (curveE), and 0.70 (curve F) of maximum strength, the oxygen content isobserved to reduce. At the start of crystal growth the magnetic fieldwas not applied. This assisted in growing out to the required diameter.After the magnetic field is applied there is a drop in oxygen content.This is most marked for the 0.70 magnetic strength curve where themagnetic strength is started at about 0.12 fraction solid and is fullyon at about 0.16 fraction solid. Alternatively the entire crystal fromseed-on may be grown under the magnetic field.

We claim:
 1. A method of growing semiconductor crystalline materialscomprising the steps:providing in a crucible a melt of the material tobe grown as a crystal, dipping a seed crystal into the melt, withdrawingand rotating the seed crystal from the melt, maintaining a temperaturegradient between the melt and seed crystal, whereby a single crystallinematerial is grown from the melt, during said withdrawing and rotatingstep, providing a magnetic field that is substantially rotationallysymmetrical about the axis of rotation of the crystal and with acomponent of field parallel to the axis of crystal rotation that is lessthan 500 gauss at the interface between growing crystal and melt, with avalue of magnetic field above 500 gauss at other parts of the melt, andmaintaining this distribution of magnetic field during growth of thecrystal, whereby forced convective flows in the melt adjacent thecrystal/melt interface are substantially undamped, whilst convectiveflows in other parts of the melt are damped.
 2. The method of claim 1wherein the component of field parallel to the axis of crystal rotationis less than 200 Gauss at the interface between growing crystal andmelt.
 3. The method of claim 1 wherein the crucible is rotated about itsaxis during growth of the crystal.
 4. The method of claim 1 wherein thecrucible is raised during growth of the crystal.